1. Field of the Invention
Aspects of the present invention relate to a fuel cell system. More particularly, aspects of the present invention relate to a fuel cell system in which fuel cell efficiency does not deteriorate in the course of restarting after being left in an extreme environment (i.e., exposed to a high or low temperature) and an operating method thereof.
2. Description of the Related Art
A fuel cell is a power generation device for generating electric energy through an oxidation reaction of a fuel and a reduction reaction of an oxidizing agent. Fuel cells may be of many types, including a polymer electrolyte membrane fuel cell and a direct methanol fuel cell.
A polymer electrolyte fuel cell receives a reforming gas that is reformed from liquid fuel or gas fuel, and an oxidizing gas, such as oxygen. The polymer electrolyte fuel cell generates electric energy through an oxidation reaction of the reforming gas and a reduction reaction of the oxidizing gas. A direct methanol fuel cell receives liquid fuel and air, and generates electric energy through an oxidation reaction of the fuel and a reduction reaction of an oxidizing gas. Since the direct methanol fuel cell has simple constituent elements, it is often used as a portable power supply.
A fuel cell system employing the fuel cell includes a fuel cell power generator for generating electric energy through an electrochemical reaction between hydrogen and oxygen. The fuel cell system further includes a fuel supplier for supplying fuel containing the hydrogen to the fuel cell power generator and an oxidizing agent supplier for supplying an oxidizing gas containing the oxygen to the fuel cell power generator.
However, conventional fuel cell systems have a problem in that fuel cell efficiency is deteriorated in the course of restarting after the systems are left in an extreme environment where the temperature is extremely high or low. Particularly, since direct methanol fuel cells are widely used for portable power supplies, it is more likely that they are exposed such environments; and thus, the fuel cell efficiency deteriorates.
To resolve such problems, one conventional technology suggests a structure of applying a reverse current introduced to the fuel cell power generator. The fuel cell system of the conventional technology quickly recovers the fuel cell efficiency during restarting by supplying a reverse current to a fuel cell power generator even though the fuel cell power generator is left in an extreme environment where the temperature is extremely high or low.
The conventional fuel cell system supplies the reverse current for a time as short as about 18 seconds or up to about 180 seconds. However, it may be possible for the reverse current to be supplied even if the fuel cell system is not left in an extreme environment where the temperature is extremely high or low. As such, the conventional fuel cell system may consume power unnecessarily when it restarts after the fuel cell power generator stops. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not necessarily form the prior art that is already known in this country to a person of ordinary skill in the art and the above disclosure is not an admission as such.